The problems that are solved by the present invention may be encountered in many situations, but they are perhaps best understood by reference to a typical and rather familiar example.
In a tavern that serves draft beer, one or more kegs of beer are kept in a cooler at some distance from the tap at the bar, and beer flows from a keg to the tap through a beverage duct or so-called beer line. To force beer out of the keg and through the beer line, the interior of the keg is pressurized with compressed air, nitrogen or carbon dioxide gas that is metered into the keg from a compressor or tank. In the case of carbon dioxide, the gas pressure in a freshly charged tank is on the order of several hundred pounds per square inch typically ten times the ambient temperature in degrees F. Although compressed air and nitrogen may be at lower source pressures, all of these gases are under substantially higher pressure than is desired in the interior of a keg, which should be pressurized only to about 20 to 30 p.s.i. In flowing from the source to the keg, the gas is therefore passed through at least one pressure reducing valve. With stored pressure gas there is usually a two-stage pressure reduction system comprising a first-stage regulator at the gas tank outlet and a second-stage regulator near the keg or at a keg manifold connected with plural kegs.
Of course pressure reducing valves are normally very reliable, but they have been known to fail. If the full pressure of gas from a carbon dioxide tank were charged into a keg as the result of such failure, the keg would explode violently. In the absence of a safety device, failure of only a first-stage regulator in a two-stage system would probably be less disastrous, but could rupture the pressure gas duct. As an essential safety precaution, therefore, it has become virtually mandatory to incorporate at least one overpressure relief device into the pressure gas system of every beverage dispensing installation of the type just described, including wine and soda dispensing systems as well as draft beer systems. Existing safety codes recommend two such devices, one located at the outlet of the primary regulator, the other at a point in the gas system that is a little distance upstream from the kegs or product tanks.
One such overpressure relief device heretofore available comprised a rupture disc of aluminum foil or the like, sealed across an opening in the gas pressure system and intended to be broken by gas pressure in excess of a predetermined value. Once ruptured by an overpressure, the disc of course had to be replaced, and this occasioned a certain amount of inconvenience. Furthermore, rupture of the disc usually resulted in loss of substantially the entire contents of the pressure gas tank. Another significant disadvantage of the rupture disc was that there was no way of testing it without destroying it.
Obviously these disadvantages of a rupture disc safety device are not present in a pressure relief valve that opens in response to pressure above a predetermined value and automatically reseats itself when pressure drops below that value. The pressure at which such a device opens can be readily checked and adjusted without destroying or harming it.
Heretofore, however, no satisfactory safety relief valve has been available for beverage dispensing gas systems, even though innumerable relief valves have been devised for other types of systems. One reason for this anomaly is that most prior pressure relief valves were intended to be mounted at a location to be protected, to be subjected to the static pressure of fluid at that location, as in the case of a steam boiler safety valve which is connected into the boiler itself. In a sense, the keg or beverage tank in a beverage dispensing system constitutes a location to be protected, but it is not practical to install a safety valve in the keg itself. Furthermore, there is also a substantial length of ducting which extends to the key from the pressure gas source, and that ducting must also be protected against rupture by an overpressure. In particular, the duct that extends between a primary and a secondary pressure regulator normally includes no plenum at which a conventional safety relief valve could be mounted.
It can be seen that a safety valve for the gas system of beverage dispensing equipment must perform a somewhat different function than a conventional safety valve, in that it must be responsive to conditions at some distance downstream from where it is mounted, and it must be mounted at a location at which there is a flow of gas past it, rather than being subjected substantially only to the pressure of non-flowing fluid. As is well known, a rapidly flowing gas tends to exert a low static pressure on surfaces along which it flows. Because of this, a conventional safety relief valve, connected in the pressure gas system of beverage equipment, would not serve to protect the system because there would be an increasing gas flow rate past it at those times when it should be opening to relieve pressure. Such a safety valve would remain closed until pressure at the downstream end of the gas duct had increased to near-equality with pressure at the source, causing a wave of static pressure to be reflected back along the duct.
A safety device for connection in the gas system of beverage dispensing equipment should desirably meet certain structural limitations, especially if it is to be adapted for retro-fitting into existing equipment as well as for incorporation into new equipment. It must of course be sturdy, inexpensive and compact. In addition, for maximum installation versatility, it should be connectable for effectively in-line gas flow; that is, it should have inlet and outlet nipples that project in opposite directions and are preferabably coaxial with one another. It will be apparent that provision for such in-line connection is not easily reconcilable with the rather unusual functional requirements imposed upon a safety valve for a beverage dispensing pressure gas system. Prior safety devices for such systems were usually arranged to bring system gas flow to a dead-end point, and there tends to be an inherent incompatability between an arrangement that provides for such dead-ended flow and one that provides for effectively straight-through or in-line flow.
The provision of coaxial, oppositely projecting inlet and outlet nipples brings another problem in its wake, particularly with respect to a safety relief valve intended to be retro-fitted into existing systems and which must therefore be connected between other units in such systems. If a particular one of the nipples must always be connected as an inlet and the other as an outlet in order for the device to perform its safety function, then there is always a chance for the device to be incorrectly installed, with possibly disastrous results. The best insurance against such a safety valve being connected backwards is to arrange it so that it is incapable of being installed backwards; that is, it should be satisfactorily connectable with either of its nipples used as its inlet and the other as its outlet.
One other requirement is substantially peculiar to beverage dispensing equipment. Any unit in such a system is susceptible to having a relatively sticky substance sprayed or spilled onto it, in addition to being exposed to other types of foreign matter that may be present in an uncontrolled environment. Obviously these possibilities have to be taken into account in the design of the device.
It will be apparent that the provision of a completely satisfactory pressure relief safety valve for the pressure gas system of beverage dispensing equipment presents a complex of problems. When these problems are fully understood, individually and in their relationship to one another, it is understandable that there has not heretofore been a satisfactory safety valve for such a system, notwithstanding the well-developed state of the art relating to safety valves generally. The need has been evident enough, but it has been far from obvious how that need could be fully satisfied.